linux-kernel - Re: [RFC PATCH 04/11] x86,fpu: defer FPU restore until return to userspace

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Message-ID: <54B572CA.9060607@redhat.com>
Date:	Tue, 13 Jan 2015 14:32:26 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	linux-kernel@...r.kernel.org, mingo@...hat.com, hpa@...or.com,
	matt.fleming@...el.com, bp@...e.de, pbonzini@...hat.com,
	tglx@...utronix.de, luto@...capital.net
Subject: Re: [RFC PATCH 04/11] x86,fpu: defer FPU restore until return to
 userspace

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On 01/13/2015 12:58 PM, Oleg Nesterov wrote:
> On 01/11, riel@...hat.com wrote:
>> 
>> Defer restoring the FPU state, if so desired, until the task
>> returns to userspace.
> 
> And yet another concern ;) Although I feel that I am totally
> confused and probably wrong.
> 
>> --- a/arch/x86/include/asm/fpu-internal.h +++
>> b/arch/x86/include/asm/fpu-internal.h @@ -382,6 +382,7 @@ static
>> inline void drop_init_fpu(struct task_struct *tsk) else 
>> fxrstor_checking(&init_xstate_buf->i387); } +
>> clear_thread_flag(TIF_LOAD_FPU); }
> 
> OK, but shouldn't (say) restore_user_xstate() clear TIF_LOAD_FPU
> too? Otherwise, can't switch_fpu_finish() restore the wrong context
> later?

I address restore_user_xstate and save_user_xstate later in the
patch series. Once I have all the code working, I should refactor
that, in order to get a cleanly bisectable series.

> Or. Perhaps switch_fpu_finish() should do nothing if fpu.has_fpu ==
> T, I dunno.

One of the later patches in the series implements that.

- -- 
All rights reversed
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